JSI Tip 6305. What are the differences between Windows XP SP1 and Windows XP XP1a?

Microsoft Knowledge Base Article 813926 contains the following summary:

This article describes the differences between Windows XP Service Pack 1 (SP1) and Service Pack 1a (SP1a). Windows XP updates are distributed in service packs. You can use service packs to help keep Windows XP current and to extend and update the functionality of your computer. On September 9, 2002, Microsoft released Windows XP SP1. On February 3, 2003, Microsoft released SP1 again as SP1a.